Sparkfun electronics sells some very inexpensive 4-digit led displays with an SPI -like interface. It would appear that these are no longer available at Sparkfun, but they are available from Hitt Consulting: http://www.hittconsulting.com

Here's the datasheet: pdf

Here's some code to run em:

unsigned long time;
int led_state;
char numStr[]="0000";  

#define CLOCKPIN  3  //change this to the pin where the clock is going in to
#define DATA_PIN   2  // change to the pin where the data wire is plugged in to
#define LED_PIN   13  // status led 


byte digit[4];
byte number[] = {  126,   //0  numerals (0-9)
                    24,   //1
                   109,   //2
                    61,   //3
                    27,   //4
                    55,   //5
                   119,   //6
                    28,   //7
                   127,   //8
                    31,   //9
                    60,   //C
                    71,   //F
                   103,   //E                    
  };

void setup() {
  Serial.begin(19200);
  time=millis();
  pinMode(DATA_PIN, OUTPUT);
  pinMode(CLOCKPIN, OUTPUT);    
Serial.println("Ready");
}

void dispDigit(byte dig, byte flash) { 
   for(int i = 0; i < 8; i++) { 
      digitalWrite(DATA_PIN,  (dig & B10000000));    // select high bit 
      dig = dig << 1;                                // next bit 
      digitalWrite(CLOCKPIN, HIGH);                  // clock high 
       if (flash) delay(1);                          // wait to set display on
      digitalWrite(CLOCKPIN, LOW); 
   } 
} 

void numParse(int num){
  itoa(num, &numStr[0], 10);  
    int z=0 ;
    if (num<1000){z++; digit[3]=number[0];}    
    if (num<100){z++; digit[2]=number[0];}
    if (num<10){z++; digit[1]=number[0];} 
    if (num<1){z++; digit[0]=number[0];}

  for (int g=0; g < (4-z); g++) digit[g]=number[numStr[3-g-z]-48]; 
} 

void loop() { 

if (millis() - time >100){  
numParse(millis()/100);
for (int i=0; i<3; i++) dispDigit(digit[i],0);
dispDigit(digit[3],1);     
time=millis();
led_state=!led_state;
digitalWrite(LED_PIN, led_state);
}

}